1. Field of the Invention
The present invention relates to a sheet for changing the travel direction of light, and more particularly to an optical sheet which can prevent cause significant problems when it is superimposed on another member. The present invention also relates to a surface light source device, a transmission type display device and a light emitting device each having such a useful optical sheet. Further, the present invention relates to a mold for molding such a useful optical sheet and to a method for producing the mold.
2. Description of Related Art
A surface light source device which, together with a transmission type display unit (e.g. liquid crystal panel), constitutes a transmission type display device, includes a light source and a number of optical sheets (optical films) for changing the travel direction of light from the light source, as disclosed e.g. in JP 8-304608A, U.S. Pat. No. 7,072,092 and JP 2008-544303T.
In general, the number of optical sheets include an optical sheet having a function (light diffusing function) to diffuse light from the light source, thereby blurring or obscuring the image of the light source, an optical sheet having a function (light condensing function) to change the travel direction of light so that the angle (exit angle) between the direction of the exiting light and the front direction becomes smaller, thereby enhancing the front direction luminance, etc.
Among optical sheets having a light condensing function is widely used an optical sheet having linearly-extending unit prisms (unit optical elements) arranged in a direction perpendicular to the longitudinal direction of the prisms (so-called linear array arrangement) (JP 8-304608A and U.S. Pat. No. 7,072,092). The unit prisms of the optical sheet each typically have a triangular, ellipsoidal or circular cross-sectional shape. Such unit prisms thus have ridge lines extending in the longitudinal direction. An optical sheet, having unit prisms each having the cross-sectional shape of an isosceles triangle (typically an isosceles right triangle) which is symmetrical with respect to an axis extending in the front direction, is currently considered to be capable of providing the highest level of front direction luminance.
Various problems may arise when two optical sheets are superimposed on each other, or when the light exit side surface (light outgoing side surface) of a surface light source device and the light entrance side surface (light incident side surface) of a transmission type display unit are superimposed on each other and optically unified. Specifically, a bright spot may be produced in a region where two members are superimposed on each other due to the phenomenon (so-called “direct passage of light”) of light passing through the region without being subject to an optical action. A stripe pattern (like so-called “Newton's rings”) may be produced in a region where two members are superimposed on each other. Further, a wetting pattern (also called “wet-out”) like staining with a liquid may be produced in a region where two members are superimposed on each other. It has been found through the present inventors' studies that such problems arise not only when the surfaces of two members make plane contact with each other but also when the unit prisms of the above described optical sheet, having the ridge lines, make contact with another member.
It is conventional practice to dispose a light diffusing sheet between an optical sheet and a transmission type display unit in order to reduce such problems. On the other hand, with a view to producing thinner display devices and lowering the production cost, there is a demand in these days for solving the problems without using a light diffusing sheet.
JP 8-304608A discloses another method to address the problems. In the optical sheet disclosed in JP 8-304608A, the ridge lines of the unit prisms undulate moderately in a curved line in the height direction. Accordingly, the optical sheet makes contact with another member only in the locally high ridge portions of the unit prisms. The method of JP 8-304608A thus decreases the contact area between the optical sheet and another member so as to reduce problems due to contact between them. However, in view of influence on the quality of display images and also of the production cost, it is not possible in actual mass production to produce an optical sheet with unit prisms having largely varying heights. Thus, the unit prisms having moderately undulating curved ridge lines make contact with another member in a larger area than has been expected, making it impossible to fully solve the problems. Further, the contact area between the optical sheet of JP 8-304608A and an adjacent member can significantly increase when the contact pressure between them increases due to deformation of the optical sheet caused e.g. by moisture absorption or thermal expansion. For these reasons, it is not practically possible for the optical sheet of JP 8-304608A to substantially solve the above problems.
In the optical sheet disclosed in U.S. Pat. No. 7,072,092, the unit prisms each undulate continuously and moderately in a direction parallel to the sheet plane. This method can reduce interference between the arrangement of the unit prisms and the arrangement of pixels in a transmission type display unit. However, the unit prisms are generally arranged closely (at short repetition intervals) in order to prevent “direct passage of light”. Thus, because of constraint by adjacent unit prisms, the degree of horizontal undulation of each unit prism cannot be made sufficiently large. Furthermore, the optical sheet of U.S. Pat. No. 7,072,092 is not effective to solve the problems of “direct passage of light” and “wet-out”. For these reasons, it is not practically possible for the optical sheet of U.S. Pat. No. 7,072,092 to substantially solve the above problems.
On the other hand, JP 2008-544303T discloses an optical sheet having a two-dimensional arrangement of large number of irregular prism blocks. The large number of prism blocks is arranged irregularly on a base and, in addition, the prism block construction, such as the shape and the dimensions, vary irregularly among the large number of prism blocks. Because of the irregularity of the surfaces of the irregular prism blocks, the optical sheet disclosed in JP 2008-544303T is expected to be capable of reducing the problems associated with its contact with another member. However, the optical effect of the optical sheet of JP 2008-544303T is completely different from that of an optical sheet having a linear array of linearly-extending unit prisms; in particular, the effect of condensing light into a particular direction component is significantly lower. Thus, the optical sheet of JP 2008-544303T cannot be simply replaced with an optical sheet having a linear array of linearly-extending unit prisms e.g. in a surface light source device. Further, as described in JP 2008-544303T, the optical sheet is generally produced by molding using a mold. In the production of a mold for the optical sheet of JP 2008-544303T, it is necessary to form in the mold a large number of recesses corresponding to the prism blocks having irregular shapes and arranged irregularly. The production of such a mold is very difficult and incurs a considerably increased production cost. This leads to a considerably high production cost of the optical sheet of JP 2008-544303T compared to an optical sheet having a linear array of linearly-extending unit prisms.
As described hereinabove, the problems that arise when superimposing an optical sheet having unit prisms on another member have not been fully solved as yet.
Devices, including a light source and an optical sheet(s) which receives light from the light source, are widely used as light emitting devices having various light emitting functions (including illuminating functions), for example, a lighting device (illuminating device), an indicator light, a sign (signboard), a mark, etc. The above-described problems with a surface light source device or a display device may likewise arise in such a light emitting device when the device includes a plurality of optical sheets or when an optical sheet of the device makes contact with a member adjacent to the device.
Further, in a surface light source device, two optical sheets having unit prisms are often used such that they are superimposed on each other with their ridge line directions intersecting each other. In the actual use of a surface light source device including such two optical sheets, the front direction luminance is sometimes considerably lower (by a few percent) than an expected design value. Nowadays, when environmental issues are attracting continued attention, enhancement of the energy efficiency in a surface light source device is an important problem. It is favorable if this problem can also be solved by the present invention.